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THE PRE-PRE CHECK
It's a good practice to be sure there are no vacuum leaks cause a vacuum leak will hinder idle adjustments and will mess up the fuel mixture and delivery.
Some Things That Can Cause Vacuum Leaks
- PCV Valve
- Intake manifold and Carb Gaskets
- Vacuum Lines
- Faulty Thermovalves, check valves, or Solenoids
- Leaky diaphragms
- Leaky O-rings or internal carb passages
Also make sure your float level is set correctly.
Always check oil and radiator/coolant fluid levels before starting the car. This is good in it's own way but for the sake of idle adjusmtent, the themovalves require coolant in order to operete parts on the carb correctly.
Also an overheating car can have idle and driving problems, and a carb not receiving coolant can have winter driving problems.
Another possible problem is a loose booster venturi, click here to find out how to check them.
I adjust my carb the way listed below. This may not be how the manual/book does it.
Just to let you know I don't have these things on my carburetor:
- Fast Idle
- A/C Idle boost
However I'll include adjustment for the A/C idle boost.
I also don't have more then that on my carburetor cause I removed many parts but for the sake of discussion I'll add on parts and show you when I adjust them.
Remember most adjustments to idle speed or ignition timing are made with the car fully warm. The only cold adjustments are the FAST IDLE and CHOKE.
I start off by setting the base ignition timing, which is the Ignition timing without vacuum or rpm advance / retard. So you need to disable / disconnect both vacuum advances ; Vacuum lines #2 & #25.
When disabling the vacuum advance you disconnect the vacuum lines to the advance diaphragm... these two vacuum lines need to be sealed so that they do not bleed vacuum and cause a vacuum leak while disconnected.
set the ignition timing with a timing light aimed at the flywheel or plate on the torque convertor.
Usually you won't need to disable #25 because all idle adjustments should be made with the car fully warm. When the car is fully warm there is no vacuum running thru #25 to the distributor vacuum advance port.
Also the first idle adjustments should be made with the A/C and Accessories OFF.
With everything off and the car fully warm, prevents the accessories, the choke, and cold start equipment form interfering in the idle adjustment process.
The Prep , If you can't or don't know how to adjust ignition timing then proceed to STEP-THREE. However if ignition is your problem and you choose to skip adjusting ignition, then you'll just be masking your ignition problem with a higher idle.
Set base ignition timing ( Vacuum Advance disabled ) Refer to steps above !
. Remember, after you set the base ignition timing you need to reconnect the vacuum advance #2
Connecting #2 before or after adjusting the throttle stop screw will affect how your car will respond when there is a sharp drop in manifold vacuum.
If you connect #2 after adjusting the throttle stop screw then a sudden drop in manifold vacuum wont bother the idle.
If you adjust the throttle stop screw after #2 is reconnected then a drop in manifold vacuum can lower the idle past the throttle stop screw setting; although the idle diaphragm if connected should even out the idle.
#25 can remain disconnected.
Turn out all Idle Diaphragm screws (If you need to temporarily raise the throttle stop screw... For Example if the idle droped way below 800 rpm )
Here Are The Two Scews
... a. A/C idle boost screw
... b. Idle Diaphragm Screw ( Dashpot )
* This will keep these screws from interfering with the throttle stop screw idle adjustment.
Now it's time to adjust the idle speed
(3) Set the Throttle Stop Screw to the Best & Lowest RPM idle speed.
Usually this is about 500 - 800 RPM, but you may need it higher.
Remember it is possible to set the throttle stop too high... too high meaning the throttle plate is open too far causing the carb to come off of the IDLE CIRCUIT. When the carb is off the IDLE CIRCUIT it has a hard time pulling fuel form the idle jet, and it could cause idle problems.
3a.) Not that you should, but if you need to, now is the time to adjust the fuel mixture and see if it affects idle. I've adjusted the A/F screw on a weber carb so that the idle raises... and back off when the idle stops raising.
3b.) Readjust the Throttle Stop Screw as needed.
3d.) If you haven't already done so now is the time to connect #2 vacuum advance.
4) Turn the Idle diaphragm until you reach a stable rpm... about 1,000 rpm the MAX is 1200 rpm in neutral... adjustment past 1200 is not recommended, and you should look elsewhere to fix your idle issues.
Usually 1000 RPM in gear (automatic) or 900 - 1000 with lights turned on is a good idle speed.
There is a second segment added to the body of the A/C idle boost diaphragm that aids in idle when the car is in gear.
The vacuum port on this segment connects to vacuum line #21
The idle diaphragm adjustment and A/C idle boost is partly to help automatic cars run when in gear and also aide both manual and automatic cars in idling when the headlights and regular interior fans are turned on.
*A/C SHOULD NOT BE ON*
IF YOU HAVE A/C GO TO STEP #5
5) Keep the headlights and fans on and also turn on A/C
Adjust the A/C idle boost to raise the idle speed back up to where the idle speed was prior to the A/C being turned on.
BTW when I say turn on the A/C it should be at full blast.
After the A/C idle boost is set you're all out of adjustment screws... at least ones for idle after the cold start process.